The invention relates to the art of electrotechnology and electronics. Objects for which the invention may have practical application are components based on surface acoustic waves, such as bandpass filters with a relative bandwidth on the order of 0.1 per cent and resonators for oscillators.
Transverse-mode-coupled resonator filters are known in which there are disposed adjacent to one another on a piezoelectric substrate a plurality of one-port resonator structures, which comprise two strip reflectors and an interdigital transducer, wherein the strip reflectors enclose a plane cavity and their reflector strips are shorted by short-circuit strips, while the interdigital transducer is disposed in the plane cavity, the strip zones of the strip reflectors and the finger zones of the interdigital transducers in each one-port resonator structure forming, together with the short-circuit strips and with the collector electrodes respectively, waveguides for surface acoustic waves, and the one-port resonator structures being coupled with one another by virtue of the waveguide effect.
In a special embodiment (German Patent 19744948), two one-port resonator structuresxe2x80x94referred to therein as waveguide tracksxe2x80x94are coupled via two further waveguide tracks, which do not contain any transducers. The otherwise homogeneously metallized outer collector electrodes are subdivided into strips of different widths extending toward the transducer fingers, and into gaps located therebetween. This subdivision ensures that phase velocities with values between that in the grating regions (waveguide tracks) and that in the free zone are established in the outer collector electrodes. Wave excitation does not take place in these zones. All transducer fingers and reflector strips have the same width. This is also true of the gaps between the transducer fingers and reflector strips.
The most widely used special embodiment comprises two one-port resonator structures (M. Tanaka, T. Morita, K. Ono and Y. Nakazawa, xe2x80x9cNarrow bandpass filter using double-mode SAW resonators on quartzxe2x80x9d, 38th Annual Frequency Control Symposium 1984, pp. 286-293 [1]). The two transducers, which are operated as the filter input and output, have a common collector electrode, which is connected to ground potential. Usually two identical filters of this type are interconnected as a cascade.
The disadvantage of this embodiment is that the cascade connection leads to excessive damping, and so the insertion loss of the filter is too large.
The object of the invention is to modify the known type of transverse-mode-coupled resonator filters based on surface acoustic waves such that the insertion loss of the filters is reduced, especially by reducing the damping due to the cascade connection.
This object is achieved according to the invention with the transverse-mode-coupled resonator filter described in the claims.
The inventive resonator filter is characterized in that at least one of the one-port resonator structures differs from the other one-port resonator structures by the ratio of the finger width to the spacing of the finger centers and by the ratio of the width of the reflector strips to the spacing of the centers of the reflector strips.
The velocity of propagation of surface acoustic waves depends on, among other factors, the ratio of the finger width to the spacing of the finger centers and of the width of the reflector strips to the spacing of the centers of the reflector strips, or in other words the metallization ratio. The presence of one-port resonator structures with different metallization ratios therefore means that the propagation velocities in the finger and strip gratings of different one-port resonator structures differ from one another. Accordingly, it is possible to design resonator filters which, in those one-port resonator structures whose transducers are coupling transducers, have a propagation velocity different from that in other one-port resonator structures. Transducers of different filters connected directly to one another are regarded as such coupling transducers. This is the case, for example, in a filter cascade, in which the output transducer of the first filter is connected to the input transducer of the second filter. When two identical filters are connected in cascade, elevated energy density can be generated in the vicinity of the low-frequency of high-frequency resonance in the coupling transducers. As a result, the real conductance of the coupling transducers is increased, whereas their capacitance remains unchanged. In this way the effective coupling coefficient is increased. This is the prerequisite for reduction of the insertion loss caused by the cascade connection. The higher effective coupling coefficient causes an increase in the resonance splitting of the low-frequency and high-frequency resonance resulting from the cascade connection.
The invention can be configured expediently as follows.
In an expedient embodiment, the transducers and strip reflectors of neighboring one-port resonator structures form common collector electrodes and short-circuit strips respectively.
Those one-port resonator structures which differ from the other one-port resonator structures by the ratio of the finger width to the spacing of the finger centers and of the width of the reflector strips to the spacing of the centers of the reflector strips can also differ from one another by the spacing of the finger centers and by the spacing of the centers of the reflector strips. Nevertheless, the spacing of the finger centers and the spacing of the centers of the reflector strips can also be the same in all one-port resonator structures.
The number of one-port resonator structures can be equal to two. At the same time, the apertures of the transducers and reflectors in the two one-port resonator structures can be the same or different.
In a particularly expedient embodiment, the number of one-port resonator structures is greater than two. Thereby there is achieved the possibility that the one-port resonator structures form two groups, wherein all transducers within one group are connected to one another in parallel, one group representing the input transducers and the respective other group representing the output transducers.
Occasionally it is advantageous for gaps filled with reflector strips maintained at the same potential to be present between neighboring one-port resonator structures.
The transducers in at least one of the one-port resonator structures can differ from the other transducers by the finger polarity.
In a particularly expedient embodiment, two identical filters form a filter cascade, in which the same group is used as the coupling transducer in both filters, the coupling transducer being regarded as that group which is connected to a group of the respective other filter. In an advantageous version, the group that forms the coupling transducer comprises one-port resonator structures with the same finger and reflector-strip width as well as the same gaps between them. The ratio of the finger width to the spacing of the finger centers and of the width of the reflector strips to the spacing of the centers of the reflector strips in the group forming the coupling transducer can be larger or smaller than in all other one-port resonator structures.